Fluid catalytic cracking system and process

ABSTRACT

One exemplary embodiment can be a fluid catalytic cracking system. The system can include a reaction zone, in turn including a reactor receiving, a fluidizing stream, a fuel gas stream, a fluidizable catalyst, a stream having an effective amount of oxygen for combusting the fuel gas stream, and a feed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.12/340,291 filed Dec. 19, 2008, the contents of which are herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention generally relates to a fluid catalytic cracking systemand process.

DESCRIPTION OF THE RELATED ART

Cracking processes, such as fluid catalytic cracking (hereinafter may beabbreviated “FCC”), can produce different products, such as gasoline andlight olefins, e.g., propylene. In some instances, it is desired toproduce propylene along with the gasoline product in a single reactor.Unfortunately, often producing both products simultaneously can limitthe production of one product or the other. In addition, there may besome undesired side reactions.

One solution is to provide a separate reactor for producing lightolefins, such as propylene and ethylene. In such reactors, the feed andprocess conditions can be tailored to maximize the light olefin yield.Unfortunately, such dedicated reactors require additional capital inaddition to a reactor vessel, such as a furnace, heat exchangers, andother equipment. As a consequence, it would be desirable to minimize theadded capital expense of producing more than one FCC product.

SUMMARY OF THE INVENTION

One exemplary embodiment can be a fluid catalytic cracking system. Thesystem can include a reaction zone, in turn including a reactorreceiving a fluidizing stream, a fuel gas stream, a fluidizablecatalyst, a stream having an effective amount of oxygen for combustingthe fuel gas stream, and a feed.

Another exemplary embodiment may be a fluid catalytic cracking system.The system can include a reaction zone, in turn including a riserreceiving a stream. The stream may have an effective amount of oxygenfor combusting a fuel gas.

Yet another exemplary embodiment can be a process for combusting a fuelgas in a reaction zone of a fluid catalytic cracking system. The processmay include introducing a hydrocarbon feed, the fuel gas, and a streamhaving an effective amount of oxygen for combusting the fuel gas.

The embodiments disclosed herein can provide a reactor with conditionsdedicated to selectively convert heavier hydrocarbons into lightercompounds, such as propylene and ethylene. As such, the reactor can beprovided with a higher level of the desired catalyst for converting suchfeeds. In addition, the embodiments disclosed herein can provide asystem that eliminates additional equipment, such as a furnace. Such areduction in capital costs can improve the efficiency of modifying oradding such an additional reactor to an existing unit, or reducing theexpense of a new design.

Definitions

As used herein, the term “stream” can be a stream including varioushydrocarbon molecules, such as straight-chain, branched, or cyclicalkanes, alkenes, alkadienes, and alkynes, and optionally othersubstances, such as gases, e.g., hydrogen, or impurities, such as heavymetals, and sulfur and nitrogen compounds. The stream can also includearomatic and non-aromatic hydrocarbons. Moreover, the hydrocarbonmolecules may be abbreviated C1, C2, C3 . . . Cn where “n” representsthe number of carbon atoms in the one or more hydrocarbon molecules.Furthermore, a superscript “+” or “−” may be used with an abbreviatedone or more hydrocarbons notation, e.g., C3⁺ or C3⁻, which is inclusiveof the abbreviated one or more hydrocarbons. As an example, theabbreviation “C3⁺” means one or more hydrocarbon molecules of threecarbon atoms and/or more.

As used herein, the term “rich” can mean an amount of generally at leastabout 50%, and preferably about 70%, by mole, of a compound or class ofcompounds in a stream.

As used herein, the term “butene,” can collectively refer to 1-butene,cis-2-butene, trans-2-butene, and/or isobutene.

As used herein, the term “downstream” generally means a location spacedapart from another location in the direction of a flow of a stream. Asan example, a first point that is at a higher elevation on a riser thana second point would be downstream from the second point if an upwardflowing feed is provided at the bottom of the riser.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic depiction of an exemplary fluid catalyticcracking system.

DETAILED DESCRIPTION

Referring to the FIGURE, an exemplary fluid catalytic cracking system100 can include a reaction zone or a first reaction zone 200, astripping zone 270, a disengagement zone 300, a separation zone 340, anda regeneration zone or another reaction zone 380. Moreover, process flowlines in the figures can be referred to as lines, pipes, conduits,feeds, fluidized catalyst, or streams. Particularly, a line, a pipe, ora conduit can contain one or more feeds, fluidized catalyst, or streams,and one or more feeds, fluidized catalyst, or streams can be containedby a line, a pipe, or a conduit.

Particularly, the reaction zone 200 can include a riser reactor 210,which in turn, can have a reactor or reaction vessel 250 receiving ariser 260. Alternatively, the reaction zone 200 can include a fluidizedbed. Generally, the reaction zone 200, particularly the riser 260, canoperate at any suitable conditions, such as a temperature of about 425°to about 705° C., or preferably greater than about 650° C. In addition,the riser 260 can operate at a pressure of about 40 to about 700 kPa.Furthermore, the reaction zone 200 may be operated at a low hydrocarbonpartial pressure. Particularly, the hydrocarbon partial pressure can beabout 30 to about 180 kPa, preferably about 60 to about 140 kPa.Alternatively, the hydrocarbon partial pressure can be less than about180 kPa, such as less than about 110 kPa, or preferably less than about70 kPa. In one exemplary embodiment, the hydrocarbon partial pressurecan be about 5 to about 110 kPa.

The riser 260 can receive a variety of streams, namely a fluidizingstream 204, a fuel gas stream or fuel gas 208, a fluidizable catalyst212, a stream including an effective amount of oxygen 216, a recycledolefin stream 220, and a hydrocarbon feed 240.

The riser 260 can further define a combustion zone 230. Typically, thecombustion zone 230 receives the fluidizing stream 204, fuel gas stream208, fluidizable catalyst 212, and oxygen stream 216. The materials inthe equipment surrounding the combustion zone 230 can have specializedmetallurgy, refractory, and cleanliness specifications to handle anoxygen stream 216 having greater than, e.g., about 30% oxygen, by mole.

The fuel gas or fuel gas stream 208 can contain any suitable materialfor combustion. Particularly, the fuel gas stream 208 can include atleast one of hydrogen, nitrogen, carbon monoxide, methane, ethane,ethylene, propane, propylene, butane, and butene. The oxygen stream 216can include an amount of oxygen effective for combusting the fuel gasstream 208. Particularly, the stream 216 can contain at least about 5%,at least about 20%, at least about 29% and at least about 99%, by mole,oxygen. In one exemplary embodiment, the oxygen stream 216 can includeair.

Generally, the combustion zone 230 heats the catalyst to a suitabletemperature, such as at least above about 650° C. The fuel gas stream208 can be obtained from any suitable source, such as off-gas from theseparation zone 340, as hereinafter described, and/or carbon monoxideand carbon dioxide by-products of hydrogen purification. Different fuelgas streams 208 can be utilized depending on the desired effects.Particularly, heating the process by direct contact with a fuel gascombined with steam or carbon dioxide can minimize coking by olefins inhigh temperature heat exchangers. Also, using a fuel gas with verylimited added oxygen can potentially minimize steam partial pressure,temperature rise, and associated hydrothermal deactivation of thecatalyst, especially when operating the process at high catalystcirculation rates to minimize catalyst temperature rise. In addition,using the fuel gas stream 208 with low hydrocarbon, such as olefin,partial pressures at a high total process pressure may minimize the gascompression ratio. Alternatively, an oxygen stream 216 can have a highoxygen concentration minimizing purge losses from the process andnitrogen dilution of the reaction products.

The fluidizable catalyst 212 can include regenerated catalyst, spentcatalyst, make-up catalyst or a combination thereof Although thecatalyst 212 is depicted as being recycled from the disengagement zone300 through, e.g., a standpipe and slide valve, make-up and/orregenerated catalyst can be added or provided to the pipe 212 and/or atthe riser 260. The fluidizable catalyst 212 can be subject to reheatingand a mixture of a first catalyst having pores with openings greaterthan about 0.7 nm and a catalyst having smaller openings than the firstcatalyst. Such catalyst mixtures are disclosed in, e.g., U.S. Pat. No.7,312,370 B2. It is generally preferable that the catalyst mixtureinclude a high amount, if not all, of the smaller pore catalyst, such asZSM-5, to enhance light olefin yields. In addition, a catalyst, such asZSM-5, does not coke to a significant extent thereby removing therequirement of a regeneration zone. As such, burning in the combustionzone 230 can suffice to remove any coke build-up.

Generally, the first catalyst may include any of the well-knowncatalysts that are used in the art of FCC, such as an active amorphousclay-type catalyst and/or a high activity, crystalline molecular sieve.Zeolites may be used as molecular sieves in FCC processes. Preferably,the first catalyst includes a large pore zeolite, such as a Y-typezeolite, an active alumina material, a binder material, including eithersilica or alumina, and an inert filler such as kaolin.

Typically, the zeolitic molecular sieves appropriate for the firstcatalyst have a large average pore size. Usually, molecular sieves witha large pore size have pores with openings of greater than about 0.7 nmin effective diameter defined by greater than 10, and typically 12,member rings. Pore Size Indices of large pores can be above about 31.Suitable large pore zeolite components may include synthetic zeolitessuch as X and Y zeolites, mordent and faujasite. Y zeolites with a rareearth content of no more than about 1.0 weight percent (hereinafter maybe abbreviated “wt. %”) rare earth oxide on the zeolite portion of thecatalyst may be preferred as the first catalyst.

The second catalyst may include a medium or smaller pore zeolitecatalyst exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38,ZSM-48, and other similar materials. Other suitable medium or smallerpore zeolites include ferrierite, and erionite. The second catalystpreferably has the medium or smaller pore zeolite dispersed on a matrixincluding a binder material such as silica or alumina and an inertfiller material such as kaolin. The second catalyst may also includesome other active material such as Beta zeolite. These compositions mayhave a crystalline zeolite content of about 10 to about 50 wt. % ormore, and a matrix material content of about 50 to about 90 wt. %.Preferably, compositions can contain about 40 wt. % crystalline zeolitematerial, and those with greater crystalline zeolite content may beused, desirably if they have satisfactory attrition resistance.Generally, medium and smaller pore zeolites are characterized by havingan effective pore opening diameter of less than or equal to about 0.7nm, rings of 10 or fewer members, and a Pore Size Index of less than 31.

The total mixture may contain about 1 to about 25 wt. % of the secondcatalyst, namely a medium to small pore crystalline zeolite with greaterthan or equal to about 1.75 wt. % being preferred. When the secondcatalyst contains about 40 wt. % crystalline zeolite with the balancebeing a binder material, the mixture may contain about 4 to about 40 wt.% of the second catalyst with a preferred content of at least about 7wt. %. The first catalyst may comprise the balance of the catalystcomposition. Usually, the relative proportions of the first and secondcatalysts in the mixture will not substantially vary throughout the FCCsystem 100. The high concentration of the medium or smaller pore zeoliteas the second catalyst of the catalyst mixture can improve selectivityto light olefins.

The fluidizing stream 204 can also have the same composition orsubstantially a similar composition as a stripping gas stream 224, ashereinafter described. The fluidizing stream 204 can have any suitablecomposition, and typically can include light hydrocarbons such as C₁-C₄,as well as steam, nitrogen, hydrogen sulfide, fuel gas, carbon monoxide,and carbon dioxide. Alternatively, the stream 204 can be treated to havethe hydrogen sulfide and/or other gases removed. Generally, thefluidizing stream 204 is sufficient to lift the fluidizable catalystthrough the combustion zone 230 upwards through the riser 260 to thereaction vessel 250.

The gas and catalyst exiting the combustion zone 230 desirably have alow oxygen content. Particularly, the combustion zone 230 generally hasa sufficient residence time for combusting substantially all the oxygen.So, the oxygen content can be less than about 1%, preferably less thanabout 0.1%, by mole.

After exiting the combustion zone 230, a feed 240 can be provided to theriser. Generally, the feed 240 is preheated to a temperature of at leastabout 300° C., preferably about 400° to about 550° C., and optimallyabout 400° to about 490° C. Generally, it is desirable that thehydrocarbon feed 240 be a gas. The hydrocarbon feed 240 can include atleast about 50%, by mole, of the components as a gas. Preferably, theentire hydrocarbon feed 240, e.g., at least about 99%, by mole, is agas.

Generally, the feed 240 can have an effective amount of one or moreC₄-C₇ hydrocarbons for producing propylene. Preferably, at least about10 wt. % of the feed 240 is olefinic. In other preferred embodiments, atleast about 60 to about 70 wt. % of the feed 240 may be olefinic. Inaddition, optionally a recycle stream 220 can be provided to the riser260. The olefin stream 220 can, independently, have generally the samecomposition as the feed 240. In one preferred embodiment, the recycledolefin stream 220 can have C4⁺ olefins, preferably C4 and/or C6 olefinsfor increasing the yield of a desired light olefin, such as propylene.Afterwards, the mixture of catalyst and feed can rise upwards to the oneor more swirl arms 244 at the termination of the riser 260. The one ormore swirl arms 244 can separate one or more hydrocarbon products, suchas a propylene product, from the catalyst. Generally, although the swirlarms 244 can separate the catalyst from the hydrocarbon within thereaction vessel 250, reactions may still be ongoing due to contactbetween at least some of the catalyst and at least some of thehydrocarbon.

Afterwards, the hydrocarbon and some of the remaining entrained catalystcan enter the disengagement zone 300. Generally, the disengagement zone300 can include any suitable disengagement device, such as a cycloneseparator unit. The cyclone separator unit can include any suitablenumber of cyclones for removing the remaining catalyst particles fromthe product hydrocarbon stream. Thus, the catalyst can be separated andthrough dip legs dropped to the lower regions of a shell 140.Subsequently, the catalyst can enter a stripping zone 270 via openingsin the reaction vessel 250 where the addition of a stripping gas canstrip interstitial hydrocarbons and absorbed hydrocarbons from thesurface of the catalyst by counter-current contact. Generally, thestripping gas stream 224 is a pre-heated fuel gas or recycled gas fromthe separation zone 340 to minimize hydrothermal deactivation.Alternatively, the stripping gas can be steam. Such cyclone separatorsand stripping zones are disclosed in, for example, U.S. Pat. No.7,312,370 B2.

Afterwards, the catalyst can continue to flow downward outside the atleast one riser 260 within the reaction vessel 250 until it reaches afirst catalyst conduit 274, which can transfer catalyst from the atleast one reaction vessel 250 to a regeneration zone or another reactionzone 380. If the zone 380 is for regenerating, the zone 380 can operateat any suitable temperature, such as above 650° C. or other suitableconditions for removing coke accumulated on the catalyst particles.Subsequently, the regenerated catalyst can be returned to another FCCsystem, or at least a portion of the catalyst can be returned to theriser 260 by, e.g., communicating the regenerated catalyst with thefluidizing catalyst line 212. Any suitable regeneration zone can beutilized, such as those disclosed in, for example, U.S. Pat. Nos.4,090,948 and 4,961,907.

Alternatively, the zone 380 can be another reaction zone, such as an FCCunit that is processing heavier hydrocarbons, such as a vacuum gas oiland/or an atmospheric residue. In such an instance, the spent catalystcan be utilized for further cracking operations, and may subsequently beregenerated from that reaction zone 380.

The disengagement zone 300 can also provide the one or more hydrocarbonproducts, such as ethylene and/or propylene, which may pass to a plenum320 of the shell 140. Subsequently, the one or more hydrocarbon productscan exit via a line 264 to the separation zone 340. Optionally, the oneor more products in the line 264 can be cooled by quenching with or byindirect heat exchange with other streams to preheat them, e.g., thefeed 240 and/or the recycle stream 220, or other process streams.

Generally, the separation zone 340 can receive the products from thedisengagement zone 300. Typically, the separation zone 340 can includeone or more distillation columns. Such systems are disclosed in, forexample, U.S. Pat. No. 3,470,084. Usually, the separation zone 340 canproduce one or more products, such as an off-gas 344, which canoptionally be recycled to the riser 260 and can at least partiallycomprise the fuel gas stream 208. Other products can include a stream348 including light olefins, such as ethylene and propylene, and heavierhydrocarbons, such as C4⁺ hydrocarbons and catalyst fines, in a stream352. The stream 352 can optionally be recycled to another FCC unit thathandles heavier feeds, such as a vacuum gas oil or an atmosphericresidue. If the zone 380 is another reaction zone 380, the stream 352can be at least partially routed to that zone 380.

Thus, the system 100 can prevent carbon build-up on the catalyst andprovide process heat without furnaces and associated emissions atconditions that can minimize hydrothermal deactivation of the catalystand dilution of the light olefin product. As such, the system 100 canoperate independently of a fluid catalytic cracking unit processingheavier feeds, while taking advantage of the fluid catalytic crackingunit waste heat, if available. Particularly, such heat can be used topreheat the feed 240 to the riser 260. In the combustion zone 230, notonly can the catalyst be heated, but it also may be regenerated toremove small amounts of coke on the catalyst.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A process for combusting a fuel gas in a reaction zone of a fluidcatalytic cracking system, comprising: A) introducing a hydrocarbonfeed, the fuel gas, and a stream comprising an effective amount ofoxygen for combusting the fuel gas.
 2. The process according to claim 1,wherein the oxygen stream comprises air.
 3. The process according toclaim 1, wherein the reaction zone comprises a riser wherein the riserreceives: the fuel gas; the oxygen stream downstream of the fuel gas;and the hydrocarbon feed downstream of the oxygen stream.